Recently, as a power supply for driving a semiconductor light-emitting element such as a light-emitting diode, a power supply device which switches a DC power using a switching element is popularly used.
A power supply device of this type is often used in a lighting equipment having a dimming function that can arbitrarily control an amount of light of a light source for lighting in, e.g., a store. Such lighting equipment generally uses a four-wire dimming system as a dimming system. This is because since a large number of lighting equipments are used in; e.g., a shore, the four-wire dimming system is free from any problem of generating harmonics in input currents unlike a dimming system based on phase control, and is suited to simultaneously operating a large number of equipments.
In a power supply device of the four-wire dimming system, a dimming operation member is integrally provided to a so-called wall switch generally allocated on a wall surface. To a mechanical switch of the dimming operation member, a dimming signal generator, which supplies a dimming signal to a load via a feeder terminal, is connected. This dimming signal generator outputs a dimming signal, which is supplied to each lighting equipment. In such power supply device, when the user turns on the mechanical switch of the dimming operation member, a power supply of a lighting equipment is turned on, and a power supply of the dimming signal generator is also turned on at the same time.
No problem is posed when the ON operation of the mechanical switch turns on (power-activates) the power supply of the dimming signal generator simultaneously with power-ON (power activation) of the lighting equipment, so as to immediately output a dimming signal, thus simultaneously attaining lighting and dimming of the lighting equipment. However, when the lighting equipment is lighted on before the dimming signal is output, the lighting equipment is lighted on for a certain period in a state the dimming signal is not input before the lighting equipment is controlled to a desired light amount by the dimming signal. In general, since the lighting equipment is set to light on in a full light state when no dimming signal is input, it is lighted on in a full light state only for a moment, and then transits to a dimmed state.
Since a lighting circuit of a general electric discharge lamp forms an advanced preheating state immediately after power activation, and is set intentionally not to light on a lamp for a while, no serious problem is posed. However, in a lighting equipment which uses a semiconductor light-emitting element such as a light-emitting diode as a recent light source, a phenomenon of causing a full light state only for a moment immediately after activation due to a delay of a dimming signal readily occurs, resulting in unnatural lighting transition as the lighting equipment. Hence, the merchantability of the lighting equipment is seriously impaired.